The present invention relates to a semiconductor light-receiving device simple in structure and improved in long-term reliability, a method of manufacturing the same, a bidirectional optical semiconductor device, and an optical transmission system.
In a bidirectional optical communication system such as a fiber-optic subscriber system, an optical transmitter and an optical receiver should be installed at each of the provider's office and the subscriber's residence. Signal light from the provider is received by the optical receiver at the subscriber's residence through an optical fiber as a transmission line, while signal light from the subscriber is also transmitted to the provider's office through the same transmission line used in receiving the signal light. Normally, a semiconductor laser is used in the optical transmitter, while a photodetector is used in the optical receiver. Optical multiplexing/demultiplexing of signals transmitted in both directions is performed by a coupler.
To bring an optical module used in the optical communication system into ordinary households, it is essential to provide a lower-cost optical module that has been scaled down by reducing the component count thereof.
The following three structures have been reported as respective examples of the major components of the lower-cost optical module for use at home. Specifically, they are: an optical transmitter which achieves direct coupling between a semiconductor laser and an optical fiber by increasing working accuracy for a substrate; a coupler for performing optical multiplexing/demultiplexing which uses a PLC (planar lightwave circuit) to allow mass production of a smaller-size optical waveguide element; and an optical receiver having an optical fiber buried in a substrate and a mirror inserted in the substrate obliquely to the optical fiber such that signal light reflected by the mirror is received by a photodetector.
To compose the optical module for use in bidirectional optical communication, it is also necessary to combine the optical transmitter with the optical receiver. As examples of the integrated structure, the following two have been reported. The first structure is obtained by coupling a coupler fabricated by using a PLC to an optical fiber, a semiconductor laser, and a photodiode which have been assembled with mechanical accuracy. The first structure is reported in detail in, e.g., Optolonics, (1996) No.7, pp.139-143. The second structure is obtained by coupling a semiconductor laser directly to an optical fiber buried in a photodiode. The second structure is reported in detail in, e.g., OFC'97 Technical Digest Thl3.
Although the first structure using the PLC is suitable for mass production because of the coupler that can be manufactured by using process technology, it presents the problem of a high loss at the coupling portion between the optical fiber and the PLC.
As a solution to the problem presented by the first structure, the second structure using the photodiode with the buried optical fiber will be described with reference to the drawings.
FIG. 14 is a perspective view of a conventional semiconductor light-receiving device with a buried optical fiber. FIG. 15 shows a cross section taken along the line I--I of FIG. 14. As shown in FIGS. 14 and 15, a groove 101a for an optical fiber is formed in a main surface of a substrate 101 made of glass. An optical fiber 102 is buried as a transmission line for signal light in the optical fiber groove 101a with a UV curable resin material 103 filled in the gap between the optical fiber 102 and the optical fiber groove 101a.
Patterned electrodes 104 each made of gold are formed on the main surface of the substrate 101. A plurality of bumps 105 each made of gold are formed on the respective portions of the patterned electrodes 104 corresponding to the p-side and n-side electrode terminals of a light-receiving element. A photodiode 106 as the light-receiving element is disposed on the main surface of the substrate 101 such that the p-side and n-side electrode terminals each made of gold and formed on the light-receiving surface of the photodiode 106 are in contact with and electrically connected to the corresponding bumps 105. The UV curable resin material 103 is filled in the gap between the main surface of the substrate 101 and the light-receiving surface of the photodiode 106 to achieve bonding by using a so-called microbump bonding (MBB) process. As the UV curable resin material 103, a resin material transparent at the wavelength of signal light propagated through the optical fiber 102 and having a refractive index substantially the same as that of the optical fiber 102.
A mirror 107 for reflecting the signal light propagated through the optical fiber 102 under the photodiode 106 is also disposed in the main surface of the substrate 101. The mirror 107 intersects the optical fiber 102 at such an angle as to irradiate the light-receiving portion of the photodiode 106 with the reflected signal light, thereby allowing the photodiode 106 having the light-receiving surface parallel to the optical axis of the optical fiber 102 to receive the signal light propagated through the optical fiber 102 and reflected by the mirror 107 without high-accuracy alignment.
To constitute an optical module for bidirectional communication by using the semiconductor light-receiving device, it is sufficient to couple a semiconductor laser element directly to the terminal of the optical fiber 102 opposite to the photodiode 106 relative to the mirror 107.
Although the conventional semiconductor light-receiving device with the buried optical fiber suffers only a low signal loss because the optical fiber is used to form a waveguide without using a PLC, the UV curable resin material 103 filled in the gap between the optical fiber 102 and the photodiode 106 has a linear expansion coefficient higher than those of glass and a semiconductor, so that stresses are applied at high temperatures in such directions as to bring the bumps 105 away from each other and to bring the substrate 101 and the photodiode 106 away from each other, as shown in FIG. 15. On the other hand, stresses are applied at low temperatures in such directions as to bring the bumps 105 closer to each other and to bring the substrate 101 and the photodiode 106 closer to each other. What results is the first problem that faulty connections are likely to occur at the bumps 105 in the conventional semiconductor light-receiving device with the buried optical fiber.
Moreover, since the resin material generally softens at a temperature over the glass transition temperature Tg, the optical fiber 102 moves in the resin to vary the distance between the optical fiber 102 and the mirror 107 and hence the quantity of signal light directed to the light-receiving surface of the photodiode 106, resulting in the second problem of unstable receiving operation.
Thus, the photodiode with the buried optical fiber cannot provide long-term reliability because the resin material used in large quantity is susceptible to variations in ambient temperature and moisture, though it is reduced in signal light loss owing to the optical fiber used to form the waveguide and cost-effective owing to the ability to transmit signal light and to the resin material used in large quantity at the fixed portion.
When the photodiode and the semiconductor element are bonded simultaneously to the circuit board by reflow soldering, the conventional semiconductor light-receiving device is required not to be destroyed under high-temperature conditions at 183.degree. C. or higher in the case of using a lead-tin eutectic solder material or at a minimum temperature of a hundred and several tens of degrees in the case of using a low-melting-point solder material, so that the use of such a large amount of resin material involves trouble.
In the case of using a general-purpose lead-tin eutectic solder material in the bumps 105, as shown in FIG. 16, there occurs the third problem that a crack gradually develops at the interface between the bump 105 and the patterned electrode 104 in the course of alternately and repeatedly increasing and decreasing temperature, which leads to an electrical disconnection.